multi master comms

RMardis 4 years ago in IQANdesign updated by Gustav Widén (System support) 4 years ago 2

Just for my curiosity.

When I am setting up the gauges and what nots for the display. I notice that for the display on the MD4 I can see and display all of the sensors on any of the masters without doing a APPOUT/IN or JFIN. This tells me that those values are always being transmitted between the masters. But, if I want to use the same data values into an Application logic circuit on another master then I have to do a APPOUT/IN or JFIN. If all this data is circulating on the master or diagnostic bus anyway, why do we have to transit it again for use in the application logic? I'm building a multi master system with one md4 and two mc43.

The MD4 display pages read data that is sent on request and on change, via the Diagnostics bus. When the MD4 shows a specific display page, it requests only those signals. 

This technique is good for diagnostic info like gauges, when there are are no hard real-time requirements. 

But for machine control, you'd want messages that are sent continuously, with a timeout that makes the receiving application react quickly in case the message isn't sent. 

The Master bus is used for APPOUT/APPIN, these are good for machine control, and give you an easy way of controlling the transmit rates. The APPOUT/APPIN protocol is taking the need to deal with potential errors one step further, these messages contain additional data that makes it possible to exchange safety related data between safety certified IQAN controllers. 

A drawback of using the Master bus and APPOUT/APPIN is the overhead, with one complete CAN frame for each APPOUT/APPIN pair. In many cases, there is a need for something in between, so many designers choose to set up J1939 messages between master modules.